The present invention relates to a constant-temperature heater capable of maintaining the surface of a heat radiating plate or other similar heated member at a constant temperature.
A heating unit comprising a thermistor with a positive temperature coefficient connected in series with a power source has been used as a constant-temperature heater because its operation is free from voltage variation, its cost is low, and it has no contact. However, a heater assembly of this type has some inherent limitations. First, when the resistance of the thermistor with a positive temperature coefficient is decreased in order to attain higher heat dissipation power, the transient current is increased when a power switch is thrown. Secondly, the heat dissipation power of the thermistor per unit body volume is relatively low, so that a constant-temperature heater with a high heat dissipation power cannot be provided.
In general, the thermistor with a positive temperature coefficient used in a constant-temperature heater is pressed against a radiator plate by the force of a spring. However, the pressure exerted by the spring to the thermistor changes over a wide range from one constant-temperature heater to another because of the variation in dimensions of other component parts assembled with the thermistor and the spring. As a result, the surface temperature of the radiator plate changes over a wide range.